There is convincing evidence to indicate that the proestrous surges of LH and FSH in rat plasma are caused by a properly timed release of LHRH from the hypothalamus. It is proposed that during this event LH exerts a direct negative feedback on LHRH secretion. Experiments are designed to determine what role LH negative feedback plays in modulating the pattern of the LH surge and in ensuring the end of the LH surge during late proestrus. After the end of the LH surge, FSH remains elevated in plasma. Experiments are designed to show that this selective phase of FSH release is the result of an increase in the basal FSH secretion rate rather than a change in pituitary sensitivity to release increased amounts of FSH in response to LHRH. It is proposed that the proestrous LH surge and associated increase in plasma FSH cause a decrease in the secretion of a non-steroidal ovarian substance which permits this increased basal FSH secretion. Experiments are designed to test these hypotheses and the additional proposal that serum testosterone must be elevated prior to the onset of the LH surge for the increase in basal FSH secretion to occur. Additional studies which correlate plasma hormone levels as measured by radioimmunoassay with immunocytochemical labeling and electron microscopic examination of pituitary gonadotropes are expected to reveal that dramatic morphological changes occur during periods of coincident and selective LH and FSH release.